Browsing by Author "Allen, David"
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Item Open Access The application of electrolytic photoetching and photopolishing to AISI 304 stainless steel and the electrolytic photoetching of amorphous cobalt alloy(1986-01) Talib, Talib Naji; Allen, DavidThe results presented cover broad aspects of a quantitative investigation into the elecrolytic etching and polishing of metals and alloys through photographically produced dielectric stencils (Photoresists). A study of the potential field generated between a cathode and relatively smaller anode sites as those defined by a dielectric stencil was carried out. Numerical, analytical and graphical methods yielded answers to the factors determining lateral dissolution (undercut) at the anode/stencil interface. A quasi steady state numerical model simulating the transient behavior of the partially masked electrodes undergoing dissolution was obtained. AISI 304 stainless steel was electrolytically photoetched in 10% w/w HC1 electrolyte. The optimised process parameters were utilised for quantifying the effects of galvanostatic etching of the anode as that defined by a relatively narrow adherent resist stencil. Stainless steel was also utilised in investigating electrolytic photopolishing. A polishing electrolyte (orthophosphoric acid —glycerol) was modified by the addition of a surfactant which yielded surface texture values of 70nm (Ra) and high levels of specular reflectance. These results were used in the production of features upon the metal surface through photographically produced precision stencils. The process was applied to the production of edge filters requiring high quality surface textures in precision recesses. Some of the new amorphous material exhibited high resistance to dissolution in commercially used spray etching formulations. One of these materials is a cobalt based alloy produced by chill block spinning. This material was also investigated and electro etched in 10% w/w HC1 solution. Although passivity was not overcome, by selecting suitable operating parameters the successful electro photoetching of precision magnetic recording head laminations was achieved. Similarly, a polycrystalline nickel based alloy also exhibiting passivity in commercially used etchants was successfully etched in the above electrolyte.Item Open Access Comparative study of joining methods for a SMART aerospace application(Cranfield University, 2007-04-19) Chau, Eric T F; Friend, Clifford M.; Allen, David; Webster, John; Clark, Daniel; Goffin, KeithThe adaptive serrated nozzle (ASN) is one of the most promising concepts to help reduce the noise level generated by aero-engines. Shear between a hot air stream and ambient air at the nozzle exit creates noise. The serrated nozzle is designed to protrude into the air stream causing mixing between the two air streams reducing the noise level. Adaptive control system using shape memory alloy (SMA) actuators deploy the protrusion only when required in order to maximise fuel efficiency. The successful joining of NiTi shape memory alloy (SMA) to the titanium parent structure is critical to the development of the adaptive serrated nozzle. However, joining of SMAs to dissimilar metals is widely known as extremely difficult if not impossible. This research provides a preliminary study into the potential of using SMAs in large engineering applications such as the ASN and the development of viable joining methods for joining SMA to titanium based alloy. Five most favourable conventional joining methods were selected for experimental investigation. Results proved that the successful joining of SMA to dissimilar alloys was extremely difficult, joint failures were mainly due to the formation of brittle intermetallics at joint interfaces. The formation of these intermetallics occurs irrespective of the type of joining method and level of heat input employed. However, it has been shown that the formation of these intermetallics can be suppressed by the manipulation of the joint composition. A marked improvement in joint performance has been achieved for joints that contained no more than 25 at% nickel. Joint improvement has also been achieved through the addition of titanium at the joint, although further research is necessary to investigate the effect of titanium addition to joint performance.Item Open Access Electrochemical micromachining of microdevices from NiTi shape memory alloys(Cranfield University, 1999-05) Chen, Ta-Tung; Allen, DavidThis thesis aimed to develop a reproducible process for batch-fabricating microdevices required for MEMS and medical applications, such as micro actuators and stents, from heat-sensitive NiTi shape memory materials. Electrochemical micromachining was chosen to carry out this work. This is a non-traditional machining process involving photoresist processing and electrolytic etching which has received much attention recently for the processing of thin films. The electrolyte used was a non-aqueous solution of 5% sulphuric acid in methanol. The optimum parameters for the photoresist processing were obtained by evaluation of the thickness and exposure time of the KTFR photoresist coating. A quantitative investigation of the electrolytic etching of NiTi was carried out to study the influence of applied voltage, etch time and line width of the test pattern on the etching behaviour, e.g. etch rate, undercut, depth of etch and etch factor. The anodic polarisation behaviour of NiTi in 5% sulphuric acid in methanol was investigated under a potentiostatic control system to establish the optimum etching parameters. The materials used for the fabrication of micro actuators (required by Forschungszentrum Karlsruhe, Germany to make a prototype microvalve) were NiTi alloy thin film materials (sputtered or cold-rolled) with thicknesses ranging from 5 to 46J...lm displaying a one-way or two-way shape m:emory effect. A variety of optimised designs of micro actuator were successfully etched electrolytically at 8V. The etch rate was found to depend directly on the anodic current density. The addition of a third alloying element such as Pd or eu reduced the anodic current density and maintained a similar etch rate. However it resulted in the breaking of the films during etching due to the reduction in the ductility of the material. The materials for the micro fabrication of stents were 100J...lm thick NiTi sheets. The problem of non-uniform metal dissolution was observed. However, by adding a sacrificial etch band as a current 'robber', periodic rotation of the anode and properly adjusting the electrochemical and geometric parameters, the stents were etched successfully with improved yield and dimensional accuracy.Item Open Access Focused ion beam machining of hard materials for micro engineering applications(Cranfield University, 2009-05) Evans, R.; Allen, DavidThe Focused Ion Beam (FIB) milling of single crystal diamond was investigated and the beam drift and mill yield were quantified. The effect of water assistance on the milling of diamond was found to double the yield. The surface morphology that spontaneously forms during milling was measured and the mechanisms behind its formation investigated. The effect of gallium implantation on the diamond crystal structure was measured by x-ray diffraction. Chemical vapour deposited polycrystalline diamond (PCD) has been machined into micro scale turning tools using a combination of laser processing and FIB machining. Laser processing was used to machine PCD into rounded tool blanks and then the FIB was used to produce sharp cutting edges. This combines the volume removal ability of the laser with the small volume but high precision ability of the FIB. Turning tools with cutting edges of 39µm and 13µm were produced and tested by machining micro channels into oxygen free high conductivity copper (OFHCC). The best surface quality achieved was 28nm Sq. This is compared to a Sq of 69nm for a commercial PCD tool tested under the same circumstances. The 28nm roughness compares well to other published work that has reported a Ra of 20nm when machining OFHCC with single crystal diamond tools produced by FIB machining. The time taken to FIB machine a turning tool from a lasered blank was approximately 6.5 hours. Improvements to the machining process and set up have been suggested that should reduce this to ~1 hour, making this a more cost effective process. PCD tools with sinusoidal cutting prongs were produced using FIB. The dimensions of the prongs were less than 10µm. The tools were tested in OFHCC and the prongs survived intact. Changes to the machining conditions are suggested for improved replication of the prongs into metal. Sapphire was FIB machined to produce nano and micro patterns on a curved surface. The sapphire is part of a micro injection mould for replication of polymer parts. The comparative economics of hot embossing and injection moulding have been studied. Injection moulding was found to be the more cost effective process for making polymer parts at commercial production levels.Item Open Access The impact of tool performance on micromachining capability(Cranfield University, 2012-06) Zdebski, Daniel; Stephenson, David J.; Allen, DavidMicro-milling represents a versatile and fast manufacturing process suitable for production of fully 3D micro-components. Such components are demanded for a vast number of industrial applications including safety systems, environmental sensors, personalized medical devices or micro-lenses and mirrors. The ability of micro-milling to process a wide range of materials makes it one of the best candidates to take a leading position in micromanufacturing. However, so far it does not seem to happen. By discussion with various industrialists, low predictability of micro-milling process was identified as the major limiting factor. This is mainly because of strong effects of the tool tolerances and process uncertainties on machining performance. Although, these issues are well known, they are not reflected by the current modelling methods used in micro-milling. Therefore, the research presented in this thesis mainly concentrates on development of a method allowing a prediction of the tool life in manner of tool breakage probability. Another important criterion which must be fulfilled is the method applicability to industrial applications. This means that the method must give sufficiently accurate prediction in reasonable time with minimum effort and interactions with day-to-day manufacturing process. The criteria listed above led to development of a new method based on analytically/numerical modelling techniques combined with an analysis of real tool variations and process uncertainty. Although, the method is presented in a relatively basic form, without considering some of the important factors, it shows high potential for industrial applications. Possibility of further implementation of additional factors is also discussed in this thesis. Additionally, some of the modelling techniques presented in this thesis are assumed to be suitable for application during designing of micro end-mills. Therefore, in the last part of this thesis is presented a systematic methodology for designing of micro end-mills. This method is based on knowledge and experience gained during this research.Item Open Access Measurement and reduction of the environmental impact of industrial photochemical machining(Cranfield University, 1998-10) Ler, Leong Tat; Allen, DavidThis thesis concerns research into the environmental aspects of the photochemical machining (PCM) industry, involving measurement, analysis, benchmarking, and reducing adverse environmental impacts. The environmental audit of a PCM company found that the likely significant environmental impacts are the use of ferric chloride etchant, solvents and water. A comparison of the environmental performance of two UK PCM companies showed that there were big contrasts in etchant utilisation and solvent and water consumption, indicating that steps could be taken to reduce these impacts. A study to assess the feasibility of using laser direct imaging (LDI), a cleaner technology in photoresist imaging, found that LDI could meet the technical requirements of the PCM industry. For LDI to be economically feasible, the reliability has to be high and maintenance cost has to be low. Audit surveys of PCM companies world-wide regarding etchant utilisation and solvent consumption indicated that: (1) There is a vast difference between the performance of companies and companies that regenerate etchants were more efficient in their FeCl3 utilisation. The industrial best practice for FeCl3 utilisation is 837%. (2) Chlorination was the most popular regeneration method but most companies would use a more environment-friendly system at a higher overall cost. Regarding waste disposal, most companies sent liquid waste etchant for reclaim or recycle. (3) Half of the PCM companies no longer use solvents, and with the development of liquid aqueous-based resists, it is envisaged that PCM practitioners could eliminate the use of solvents in the near future. Lastly, an investigation into the feasibility of using oxygen gas in regenerating FeCI3 showed that the regenerated etchant could produce good quality etchings. This syst'm is also the second cheapest. Therefore, it is a good option for the PCM companies as the cost of regeneration is not too expensive and it is environment-friendly.Item Open Access Microfabrication processing of titanium for biomedical devices with reduced impact on the environment(Cranfield University, 2012-09) Gastol, Dominika A.; Allen, David; Almond, HeatherThis thesis presents research on a novel method of microfabrication of titanium (Ti) biomedical devices. The aim of the work was to develop a commercial process to fabricate Ti in a more environmentally friendly manner than current chemical etching techniques. The emphasis was placed on electrolytic etching, which enables the replacement of hazardous hydrofluoric acid-based etchants that are used by necessity when using Photochemical Machining (PCM) to produce intricate features in sheet Ti on a mass scale. Titanium is inherently difficult to etch (it is designed for its corrosion-resistant attributes) and as a result, Hydrofluoric acid (HF) is used in combination with a strong and durable mask to achieve selective etching. The use of HF introduces serious health and safety implications for those working with the process. The new technique introduces the use of a “sandwich structure”, comprising anode/insulator/cathode, directly in contact with each other and placed in an electrolytic etching cell. In this technique the same photolithography process is utilised to achieve selective etching on a metal substrate as in the PCM process. However, for the electrolytic etching stage, the inter- electrode gap (IEG) is reduced significantly from a few centimetres, as usually applied in electrochemical processes, to 4 μm. The intention behind this was to improve the current distribution experienced at the anode (Ti) during subsequent electrolytic etching. The sandwich structure was developed by deposition of a photoresist S1818 and Copper (Cu) on top of Ti. Firstly, a manual sanding of the substrate was applied in order to eliminate the oxide layers which could strongly affect a final electrolytic etching. The soft- and hard-bake stages involved in the processing of the S1818 resist were optimised to produce a stress-free Ti/S1818/Cu/S1818 structure. Ultimately a pattern would be imparted onto the S1818/Cu/S1818 that would ultimately be imparted through to the Ti layer during the last stage, electrolytic etching. In order to achieve this, a Cu electroless deposition was developed as a technique to obtain a conductive film which would act as a cathode during the electrolytic etching of the target, Ti layer. The results of the electrolytic etching of the Ti sandwich structure revealed flat-base profiles of half-etched (“half-etch” is the term used to signify an etch that does not penetrate completely through the thickness of the metal sheet) micro-holes in the Ti layer. The problem of delamination of the electroless Cu, in 10 % w/v HCl electrolyte used as an etchant, was solved by electroplating a 12 μm layer of Cu on top of the 60 nm Cu electroless deposited film. Using this technique, micro-features were achieved in Ti. The half-etched micro-holes were characterised to have an overall spherical shape corresponding to the imaged pattern and a preferred flat-base profiles (typically a raised land of material arises in conventional electrolytic etching). A series of parameters were tested in order to control the process of electrolytic etching through the Ti sandwich structure by measuring etch rate, surface roughness of the etched pattern and the etch factor. The applied current densities (CD) of 10, 15, 20, and 25 A/cm2 showed proportional dissolution to the applied current. Electrolytic etching with a CD of 20 A/cm2 demonstrated a high etch rate of 40 μm/min. and a relatively low Ra of 2.8 μm, therefore, it was utilised in further experimental work. The highest etch rate of 50 μm/min. and an improved distribution of half-etched micro-holes was achieved by the introduction of 4 crocodile connectors (2 per electrode) and mechanically stirring of the electrolyte (800 rpm) while performing the electrolytic etching. The maximum etch depth of 143.9 μm was produced in Ti when the electrolytic etching was performed at the same conditions for 3 minutes. The incorporation of ultrasonic agitation to the electrolytic etching and an electrolyte temperature of 130 C resulted in a decrease of the surface roughness of the etched micro-holes to 0.5 μm. The results of the Ti sandwich structure electrolytic etching proved the concept of minimising the IEG in order to obtain a uniform Ti dissolution on a feature scale, improved control of the electrolytic dissolution over the whole area of the sample with utilisation of the lower hazard etchant at the same time.Item Open Access Photochemical machining of brass with cupric chloride etchants(Cranfield University, 1996-02) Cakir, Orhan; Allen, DavidPhotochemical machining is a non-conventional machining process which employs photoresist and chemical etching techniques. Brass which contains 60-80% copper and 20-40%- zinc, is the most commonly etched copper alloy in the UK. It is used to produce precision parts as well as decorative items. In this study, the most widely üsed''brass (BS CZ108, ISO CuZn37) is etched with cupric chloride. The effects of etchant concentration, temperature and hydrochloric acid additions on the etch rate, undercut, etch factors and surface roughness are determined. In comparison to cupric chloride, ferric chloride and alkaline etchants were examined. As an extension to this study, the regeneration of used cupric chloride and recovery of zinc were examined. This investigation was needed because the dissolved brass in the cupric chloride affected the etch rate and surface quality of the etched brass. Therefore the etchant had to be either replaced with fresh etchant and the spent etchant disposed of, or regenerated and the dissolved metal content recovered. The regeneration/recovery process provided an economical solution to environmental considerations. Electrodialysis was used to recover zinc and regenerate cupric chloride, and the effect of various parameters (i. e. current density, temperature, catholyte solutions) were examined and a comparison of costs between electrodialysis and chlorine regeneration was undertaken. It was noticed that the electrodialysis produced etchant and catholyte wastes. In order to eliminate any waste cupric chloride or catholyte, a further investigation using cementation was undertaken. It was noted that the cementation would successfully decrease disposal of excess solutions.Item Open Access Photoetching of nickel alloys and reclamation of waste products(Cranfield University, 1995-06) White, H. J. A.; Allen, DavidThis thesis details a study into the photoetching of nickel alloys with ferric chloride and the reclamation of waste products. Reclamation occurs through the regeneration of the spent etchant and the recovery of the dissolved nickel content. Etchant regeneration is a means by which a constant etching quality can be maintained. The alternative is its replacement with fresh solution and this option entails the disposal of the exhausted solution according to correct legislative procedures. A cost comparison of both processes, was made in the early stage of this work and a model established to represent the economics of regeneration and conventional disposal (ie. not regenerating) based on the amount of nickel etched per annum. Although the dissolved nickel content will not hinder etchant regeneration, it will affect the quality of subsequent etching and the etchant will remain "contaminated". Consequently, its removal must be considered if etchant regeneration is to be implemented. Therefore, in order to establish the economical viability of nickel removal (and etchant regeneration) it was necessary to investigate potential techniques at an experimental level and to determine their technical feasibility first. Cementation and electrodialysis were investigated, the latter proving to be the more successful technique for this application. Through experimentation, it was possible to determine nickel yield data and the associated chemical and power requirements. This information was then used to calculate the 'costs associated with the process in order to extend the original model.Item Open Access Role of free hydrochloric acid in the low-cost regeneration of ferric chloride etchant by oxygen gas(Cranfield University, 2005) Jefferies, P. J.; Allen, DavidThis thesis concerns the development of a low cost and environment- friendly method for the regeneration of the most widely used aqueous etchant, ferric chloride, found in photochemical machining (PCM) facilities. In order to implement the above process a comprehensive and automated chemical analysis and control system was designed to fully investigate the key parameters which previous research has suggested play a major role in the analysis of ferric chloride. Monitoring methods have been developed and deployed in a commercial environment such that the etchant could be analysed remotely across the business network or via the internet. This level of monitoring has removed much of the `black art' previously associated with etching and has allowed proactive control of the etchant and the PCM process in general. Detailed analysis of the data captured has resulted in a clear understanding of the role the free hydrochloric acid (HC1) level plays in prolonging the life of the etchant. By keeping the free acid level high, dissolved metals remain in solution. The regeneration uses oxygen from the air, combined with surplus HCI, to regenerate the etchant within the etching machine itself. This environment-friendly system has allowed etching to continue in a very controlled way for nearly one year, during which time some 500kg of metal have been dissolved. This has totally eliminated the need to change the etchant which otherwise would have been carried out every 16 weeks. The saving to the sponsoring company has been over £7000 per year. These combined activities are considered as a major advancement in knowledge and will be of considerable benefit to the PCM industry in general. The monitoring systems alone would significantly benefit any PCM company by reducing rejects and improving product quality and productivity.Item Open Access Strategy development for special operations force logistics(2000-12-31T00:00:00Z) Moore, David M.; Allen, David; Antill, Peter D.This article highlights the specific logistics problems which special operations forces have, given their highly specialist nature and the factors that act upon them such as external (political, economic and national culture) and internal influences as well as the changing face of warfare. It will examine the ways in which special operations forces are currently supported logistically and will go on to consider the ways by which commercial 'best-practice' could be used to enhance the mission effectiveness of forces employed in this role.